This invention relates to the field of measurement of orientation and location of magnetic recording sliders within a head stack assembly.
In a hard disk drive there are one or more disks rotating on a common spindle, which store magnetic information on their surfaces. For each magnetic surface a suspension or Head Gimbal Assembly (HGA) is provided. A HGA consists of a slider, a flexure, a load beam, a mount plate, and electrical leads, which carry signals to and from the head. The slider has a read/write head, which magnetizes small areas or domains on the surface of the disk. Typically, these magnetized areas are organized in the form of tracks (e.g., concentric tracks). The slider also has an air bearing surface (ABS) fabricated on the side facing the disk. The shape of the ABS generates a pressure distribution underneath the slider as air is dragged between the slider and rotating disk; this pressure distribution tends to push the slider away from the disk. The slider is connected to the load beam via a flexure, which is a flexible member that allows the slider to pivot in the pitch (transverse axis) and roll (longitudinal axis) directions. The load beam imposes a counteracting force onto the slider to balance the pressure force. The counteracting force is called the xe2x80x9cgram loadxe2x80x9d. A method to support the HGA""s over the disks is to attach them on a single metallic piece with multiple arms. The metallic piece commonly known as an actuator is pivoted around a bearing so that the magnetic read/write elements on the slider reach various areas of the magnetic surface of the disk. A set of HGA""s mounted on an actuator is called a Head Stack Assembly (HSA). The actuator arm has a sufficient length such that the mounted HGA""s reach to the very inside tracks of the disk. Every HGA within a HSA needs to be precisely aligned relative to each other to ensure that all sliders can access the maximum allowable area on the disk. The symmetry axis, at which each HGA emanates radially from the pivot axis of the actuator is called the longitudinal or x-axis. The y-axis as the second axis is perpendicular to the x-axis and parallel to the disk surfaces. The magnetic elements of all sliders in the HSA need to be aligned to the same y-position to a close tolerance. Any deviation from the specified y-position is called y-misalignment. It is caused either by misalignment of the slider on the load beam or misalignment of the HGA on the actuator arm. One of the objectives of the current invention is to measure any y-misalignment of the sliders.
The distance between the read/write head and the disk surface is called the xe2x80x9cflying heightxe2x80x9d. It is typically below 100 nm. The main factors that affect the flying height are: the gram load, the nature of the ABS, the mounting of the slider on the flexure, the mounting of the HGA on the HSA, and the Roll Static Attitude (RSA) of the slider. RSA is a tilting of the slider around the x-axis in a free state before it is flying over the disk and can change during the HSA assembly process.
Magnetic recording is a xe2x80x9cnear-fieldxe2x80x9d process, whereby the reading and writing by the read/write head occurs in close proximity to the disk surface. To increase the areal density, it is essential to reduce the average flying height and flying height variations of the sliders, which hold the magnetic recording heads. Unintentioned alterations of the RSA and the y-misalignment during the manufacturing of head stack assemblies change the aerodynamic properties of the air bearing surface and the even distribution of the gram load thereon. RSA and y-misalignment of the sliders occur especially during the handling and the swaging process by which the HGA is attached to the actuator. Both result in flying heights and flying height variations that can exceed the predetermined limits. To minimize these alterations, tight manufacturing tolerances and a reliable quality control are necessary.
U.S. Pat. No. 5,588,200 for instance describes a method and tool to examine individual suspension assemblies or HGA""s. In the disclosed method an auto-focus vision system takes perpendicular distance measurements of several reference points of the air bearing surface of one slider. These reference points define a plane whose orientation and location are compared with control parameters. This method is space-consuming, because it requires the user to access the air bearing surface along at least one perpendicular or near-perpendicular direction. The close distance between the sliders in a head stack assembly make the air bearing surfaces inaccessible for the auto-focus vision system. The disclosed method is therefore of no use for post-assembly control.
U.S. Pat. No. 5,636,013 discloses methods to take measurements on the HGA level of orientation and z-location of sliders or suspension parts close to the slider. The z-location is recognized as a distance perpendicular to the air bearing surface of the slider. The reflection angle of a laser beam directed onto an optically reflective surface is thereby used to retrieve information about the sliders orientation. The z-location is measured simultaneously by an autofocusing system. This method is space-consuming, because it requires the user to access the air bearing surface along at least one perpendicular or near-perpendicular direction. This method is not useable for measurements on a head stack assembly for the same reasons as described under US. Pat. No. 5,588,200.
The general use of the air bearing surface as reference plane for control measurements is limited by additional incorporated three dimensional features. The air bearing surface controls the floating performance of the slider on the air stream driven by the spinning disk surface. It has more and more channels, pads and other three dimensional features added as the state of the art advances. Hence, the quality of the reflection is reduced. The reduced reflection quality in addition to limited available space between sliders within a head stack assembly make the direct optical access to the air bearing surfaces for maeasurements in the head stack assembly highly impractical.
An instrument based on U.S. Pat. No. 5,636,013 and marketed by Brumco Inc. performs the measurements on sliders within a head stack assembly. The instrument uses a miniature mirror placed between two air bearing surfaces. The mirror deflects a laser beam onto the air bearing surface, captures and transmitts the reflection via an optical lens system onto a scale. There it can be inspected. In the practical use of this instrument the variation of the measurement results exceeds the orientation and location tolerances of the sliders to be analyzed.
There is a need for a reliable and precise method and apparatus to measure the amount of RSA and the y-misalignment of each individual slider within the finished head stack assembly. The method and tool needs to be utilized for intermediate quality control within an industrial disk manufacturing. This application discloses such a solution.
Accordingly, it is a primary object of the present invention to provide an apparatus and a method to measure the y-misalignment and roll static attitude of sliders within a head stack assembly.
It is a further object of the present invention to utilize a geometric element on the deposited end of the slider with a predetermined position and orientation to provide a reference for the measurement process.
It is another object of the present invention to provide a fixture having an assisting mechanism for aiding in the measurement process.
It is an additional object of the present invention to provide an automated apparatus to systematize the measurement process.
The above objects and advantages, as well as numerous improvements attained by the apparatus and method of the invention are pointed out below.
The method and apparatus or tool for measurement of roll static attitude and y-misalignment of a magnetic recording slider within the head stack assembly comprises one or more reference elements placed on the deposited end of the magnetic slider and a fixture to hold the head stack assembly in a feasible measurement position on an automated measurement instrument. One method to fabricate reference elements is to lithographically align and co-sputter them together with the lapping guides that define the orientation of the air bearing surface of the slider during the ABS lapping process. Prior art measurements are made directly on the air bearing surface which is not easily accessible at the HSA level. In our invention, measurements are made using reference elements on the deposited end of the slider, which is easily visible even at the HSA level.
The automated measurement instrument in its preferred embodiment has a video camera, a focusing device, a pair of motorized stages, an oblique light source, and a computer. The motorized stages move the fixture with the clamped head stack assembly in a plane perpendicular to the optical axis of the video camera, bringing each slider in turn into the field of view of the camera. The computer is connected to the video camera and the motorized stages and controls them in a predetermined way.
During the calibration process, the motorized stages bring a reference edge of the fixture into the field of view of the camera. The reference edge is parallel to the axis of the bearing shaft of the head stack assembly and perpendicular to a reference orientation of the nominal roll position of the sliders in the head stack assembly. The orientation of the reference edge remains usually constant during the continuous operation of the apparatus. The reference edge has to be measured only in situations, when the camera position has changed, or when the fixture cannot be repeat ably remounted on the motorized stages within an angular tolerance that is smaller than the resolving limit of the described measurement system. The measurement process recognizes an angular difference between the reference orientation and the actual angle of the slider. The angular difference is referred to as the roll static attitude. The measurement process follows consecutive steps. To make a preadjustment on the apparatus for the predetermined field of view a precision machined reference part for example can be placed in the apparatus instead of an actual head stack assembly. The actual process begins with the clamping of the head stack assembly onto the fixture, which is afterward arrested on the motorized stages. The first magnetic recording slider is positioned with its reference element under the video camera, which takes an image of it. The location of the reference element in the field of view is found by a pattern recognition process using normalized correlation that is well known in the field of computerized machine vision. The roll static attitude and the position of each slider along the y-axis is consecutivly determined. Relative to the location of the reference element, two selected areas of the captured image are processed by the computer to determine a first approximation of the orientation of the reference element. This is accomplished by determining the position of the average edge location in each of the two selected areas, then determining the angle between those two positions relative to the reference edge. The first approximation is used in a second refined processing operation to define a starting point and to analyze a narrow fraction of the captured image. The used algorithm is similar to that disclosed in U.S. Pat. No. 5,136,660. Therein an algorithm is described for high precision measurement of angular orientation of edges. In this algorithm, a response function (maximum gradient of the Radon projection) is produced as a function of a number of test angles. The patented algorithm is applied in the current disclosure on the angle determined during the first approximation. The maximum of the response function corresponds to the angular orientation of the edge in the image under analysis. The reference element is preferably made from sputtered metal having a grain size very small in comparison to the pixel resolution of the imaging device. In lieu of a sputtered reference element, existing geometrical features on the slider such as electrical leads made from plated copper may be used; these, however, tend to have a large grain size and degrade the accuracy of the measurement. This degradation may be partially offset by using computationally more intensive algorithms as described in this document.
The reference element is preferably made from deposited copper lead with a grain size, that is relative large in respect to the pixel size of the captured image. As a result, the response function can be jagged. To generate a smooth response function, a parabolic fit is added to the patented algorithm, whereby the angle corresponding to the peak of the parabola is the final output of the algorithm.
In a following step the computer actuates the motorized stages to move the fixture with the head stack assembly until the reference element of the adjacent magnetic recording slider is placed under the video camera and the analyzing process can be repeated.